Optical Trapping Excerpt

Optical trapping refers to the use of lasers (i.e. light) in providing an attractive or repulsive force to move microscopic objects. This brief aims to discuss the physics behind this phenomenon and its applications in a manner accessible to A-level physics students.

Radiation pressure The first underlying principle behind optical trapping is radiation pressure, which in simple terms, is the fact that light carries momentum.[1] In this instance it is convenient to think of light as a stream of tiny particles (i.e. photons). Although photons have no mass they travel at the speed of light which enables them to have momentum (the details of which are discussed further in Box 1). [1] In order for photons to change direction a force must act upon them. With this in mind whenever light alters its course (reflection and refraction being good examples) it experiences a change in momentum and so to, does the object it interacts with, for example a mirror. This change will be equal in magnitude and opposite in direction, in accordance with Newton's third law. You won't notice the effect of such a change on large objects but on small objects the movement is quite measurable. [1,3]